1. Field of the Invention
The present invention relates to an exposure photomask used for manufacturing semiconductor devices such as ICs and LSIs, and an apparatus for exposing a semiconductor wafer with the photomask.
2. Description of the Prior Art
A number of reduction projection type steppers have recently been used as exposure apparatuses for performing micropattern transfer with high alignment precision in IC and LSI production lines. According to an exposure apparatus of this type, a semiconductor wafer coated with a photoresist film is exposed with a 1/5 or 1/10 image of a circuit pattern drawn on a reticle or a photomask which is reduced by a reduction projection lens. The reduced image of the circuit pattern is small with respect to the size of the wafer. For this reason, the wafer is placed on an X-Y stage and stepped by a predetermined pitch, thereby repeating projection of the reduced image. The exposure operation is referred to as a so-called "step-and-repeat" system, and the exposure apparatus of this type is called a stepper. In the manufacture of ICs and LSIs, in general, several circuit patterns must overlap and are subjected to exposure. The stepper comprises a means for accurately aligning a device region (to be referred to as a chip hereinafter) including a circuit pattern (i.e., an acutal element pattern) formed on the wafer with a new circuit pattern subjected to the overlap exposure. Several alignment techniques are considered. The most reliable technique is called a through-the-lens (TTL) system for aligning a reticle with a wafer through a projection lens. In particular, the die-by-die technique is performed as follows in the TTL system. Alignment marks are formed at a peripheral portion of the reticle between the circuit pattern and the edge thereof. The marks on the wafer are reverse projected on the reticle by the projection lens. Positional errors between the mark images and the marks on the reticle are detected so as to slightly move the wafer and the reticle relative to each other and eliminate the positional errors. According to the die-by-die technique, alignment can be performed in units of chips, thereby eliminating changes in size of elements which are caused by wafer processes (e.g., etching and diffusion) and hence obtaining highly precise alignment.
An observation apparatus is proposed as a TTL alignment means in U.S. Pat. No. 4,402,596. According to this apparatus, microopenings (i.e., the marks of the reticle) formed in the peripheral portion of the reticle are illuminated, images of the microopenings are formed by the projection lens on the wafer, and the aligning state between the microopening images and the marks on the wafer is observed above the reticle through the projection lens. In the observation apparatus, part of the microscope objective lens and a reflecting mirror are integrally movable. In this case, the microopenings in the reticle are formed in the peripheral portion of the reticle between the rectangular region (i.e., the pattern region) and the edge of the reticle. The marks on the wafers which are to be aligned with the microopening images are also formed in a peripheral portion of the wafer between the circuit pattern and the edge of the chip. When the microopenings and the marks of the wafer are observed by the observation apparatus, part of a metal member for holding the objective lens and the reflecting mirror extends to a position where part of the pattern region of the reticle is shielded. During exposure, the metal member must be withdrawn to a position where the pattern region is not shielded. This indicates that TTL die-by-die alignment requires a long time, thereby decreasing productivity (i.e., throughput) of ICs and LSIs. In addition, since the part (metal member) of the alignment means must be moved to shield or not to shield the circuit pattern, the mechanism is complicated as a whole. As a result, it is difficult to maintain uniform alignment with high precision.